Heavy snowstorms had just passed over Mammoth Mountain on April 6, 2006, when ski patrollers went out to inspect the slopes and move safety fences around a volcanic vent. Three of them, James Juarez, John “Scott” McAndrews and Charles Rosenthal, died that day after a drop into a snow cave filled with carbon dioxide, a gas that comes up from magma beneath the mountain and has no color or smell.
The deaths remain the clearest human toll of a slow-moving volcanic process that the U.S. Geological Survey has tracked in the Long Valley area of eastern California for decades. The Long Valley Observatory monitors the area, and the USGS hazard summary places the mountain inside a caldera that has had frequent earthquake swarms since 1980.
Magmatic Gas Since the 1989 Earthquake Swarm
The carbon dioxide on Mammoth Mountain is magmatic in origin, according to the USGS field guide to the mountain. It began leaking after a swarm of small earthquakes in 1989, which USGS scientists think opened cracks as magma rose and let trapped CO2 move up along faults. Trees started dying, and by 1994 USGS soil surveys had confirmed concentrations lethal to roots.
At Horseshoe Lake, on the mountain’s flank, the USGS field guide to the site reports that CO2 makes up from 20 to 95 percent of the soil gas in the affected zones, against a normal level of about 1 percent or less. Over 100 acres of trees are dead or dying there, with roots suffocated and unable to absorb nutrients. A USGS fact sheet on the tree kill put the mountain’s total CO2 emission rate at close to 300 tons a day when it was written, and the same sheet attributes the dieback to gas escaping through faults that magma had cracked open in 1989.
The April 6, 2006 Snow Cave Deaths
The USGS describes the accident in a single sentence: three members of the Mammoth ski patrol died in April 2006 when they dropped into a snow cave while moving the safety fences. Warm gases had melted the snow from below, forming a large cave that could not be seen from the surface and held toxic levels of carbon dioxide.
Trade-press coverage of the state investigation fills in the people. EHS Today’s report on the Cal/OSHA citations identifies the dead as Juarez, 35, McAndrews, 37, and Rosenthal, 58, who died attempting a rescue, and says all three died of oxygen deprivation from elevated CO2. Twenty more employees who joined the rescue needed hospital treatment.
Cal/OSHA assessed $49,865 in penalties across eight violations, including two serious citations of $18,000 each for training deficiencies. Acting Chief Len Welsh said the resort “failed to have proper procedures in place to evaluate hazards associated with volcanic fumaroles and failed to provide training to employees.” The agency also listed missing warning signs, insufficient atmosphere testing and the absence of rescue respirators. Mammoth Mountain called the event a tragedy that broke the hearts of its staff and said it would appeal the citations.
Heavier-Than-Air Gas and the Body’s Limits
Carbon dioxide is heavier than air. The USGS notes that it collects in depressions, snowbanks and enclosed spaces, and its Horseshoe Lake guide quotes the CDC in saying that breathing air with more than 7 percent CO2 can cause unconsciousness and death. The older USGS fact sheet gives a higher figure, saying air with more than 30 percent can very quickly cause unconsciousness and death, and warns about poorly ventilated cabins, tents and snow caves.
The federal workplace standard sits much lower. The CDC’s NIOSH page on carbon dioxide sets the immediately dangerous to life or health level at 40,000 ppm, or 4 percent, and its recommended exposure limit at 5,000 ppm averaged over a shift. Because the gas has no odor, a worker cannot rely on smell to detect it. A concentration that is invisible, odorless and heavier than air is the profile of a hazard that collects in a pit and goes unnoticed by anyone standing at its edge.
The snowpack matters here. The Horseshoe Lake guide notes that dangerous CO2 levels have been measured in snowpack pits inside tree-kill areas.
Monitoring From Horseshoe Lake to the Fumarole
USGS scientists track the gas year-round at Horseshoe Lake and run twice-yearly surveys of emission rates. According to the agency’s gas monitoring page, the mountain released 240 metric tonnes of CO2 a day at its July 2012 peak, modest next to erupting volcanoes that often emit over 1,000 tonnes daily. CO2 output tends to peak about two years after shallow earthquakes, a lag USGS scientists attribute to the time gas needs to travel up from the magma.
A fumarole above the McCoy station on the gondola route is fenced and monitored, and the gas there is over 98 percent carbon dioxide. The USGS hazards page for the mountain tells visitors to pay serious attention to signs warning of CO2 hazards.
The USGS reports that CO2 flux has declined gradually, though hazardous conditions persist in high-emission areas such as Horseshoe Lake, where warning signs remain posted.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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